Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by a borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, commonly referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD).
One often-collected type of downhole information is the electrical resistivity of earth formations surrounding the borehole. Though such information can be used in a variety of ways, it is often taken as an indication of the type of fluid in the formation. For example, hydrocarbon-saturated porous formations have high resistivity, while water-saturated porous formations have low resistivity.
Electrode-based resistivity logging tools can be used to measure the formation resistivity. Such tools employ an arrangement of electrodes to generate electrical currents and/or potentials, to measure the resulting electrical potentials and/or currents, and optionally to shape the associated electrical fields to focus the measurements in some fashion. The formation resistivity is then derivable from the observed relationship between electrical current and potential.
The accuracy of electrode-based resistivity logging can be affected by various factors. For example, the electrodes may react chemically with fluids in the environment to form a surface layer having a measureable (and variable) electrical impedance, altering the generated and measured electrical currents and potentials from their ideal values and reducing measurement accuracy. In tool configurations having a conductive surface near the electrode (e.g., the tool body), the formation of surface layers can be particularly deleterious if the surface layers have different impedances. Focusing is achieved by keeping the nearby conductive surface at approximately the same potential as the measurement electrode, but the difference in surface layer impedances creates an undesired potential difference in the borehole fluid immediately outside the surface layers, causing undesired current flows that impair measurement accuracy and further impair spatial resolution of the tool due to focusing impairment. The smaller the mud resistance, the larger the effect of such differences in surface layer impedance.
Thus, the materials used in constructing an electrode-based resistivity logging tool are important considerations, not only due to their tendencies to form surface layers, but also due to their effects on tool assembly and strength. Poorly-chosen materials will limit the tool's strength and prevent it from being used in a drill string or even as part of a large wireline logging assembly.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure, but on the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed with the given embodiments by the scope of the appended claims.